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4c. Preservation Metadata:
Adding Metadata
Few repositories are currently equipped to handle
the full range of preservation metadata. Creators of systems
that do accommodate it must provide answers to the following
questions:
Elements: What metadata
will be created & which attributes of the objects
will be important in the future?
OAIS provides only a conceptual framework for preservation metadata.
Several groups and institutions around the world have built on that framework
to identify and specify metadata elements. Each took a slightly different
approach and developed different metadata element specifications. Here
are the element sets proposed or used by some of those bodies: |
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OCLC’s
Digital Archive Metadata Elements |
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RLG/OCLC
Working Group’s A Metadata Framework to Support
the Preservation of Digital Objects |
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The
National Library of Australia’s Guidelines for
the Preservation of Digital Heritage (published by UNESCO) |
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The
National Library of New Zealand’s Metadata Standards
Framework (in two parts: Metadata Implementation Schema and
Preservation Metadata). |
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Cornell
University Library Proposed Metadata Elements |
Creation: Who will do the actual
work of creating the metadata?
The most efficient and accurate process starts with a common metadata framework,
used by both the Producer and the Archive. The actor who is closest to the information
to be used as metadata adds that information to the framework. For example, the
creator of a digital object knows best the technical information about the creation
of the object. Accuracy is best served if the producer adds Pre-Ingest technical
metadata to the framework. Context Information and Fixity Information are also
known at creation time. The Archive, on the other hand, holds instances of metadata
that can be shared by many objects—format standards, for example. The process
is most efficient if the Archive adds pointers or links from the many objects
to the common metadata. Then the Producer doesn't have to create redundant instances
of metadata shared by all the objects.
Manual or Automatic?
Both the Producer and the Archive may have to produce some metadata by hand,
but the goal—again, for efficiency’s sake—should be automatic
production of metadata. The METAe software
is an example of semi-automatic production of preservation metadata. For
a description of the project and information about its Metadata Engine, see RLG
DigiNews, v.6, no. 3.
Interface: How does the metadata
get from the creator to the repository?
The short answer is, “negotiation.” The long answer is that the
Producer and the Archive must make a Submission Agreement that spells out the
means of transmission, the verification process, and the process by which the
Archive can request re-transmission. In other words, the two can agree that
the files will be delivered by any means that is convenient to both. Once the
files arrive at the repository, they should be verified; the Archive can verify
the files against checksums sent by the Producer. If something goes wrong,
the Archive will want to be able to let the Producer know and be able to get
good copies of the files.
How is metadata requested and retrieved by a user?
Web browsers and HTML forms are the most common interface, although dedicated
software is a possibility. A repository can also create a process to notify
certain users of the creation or modification of objects or metadata, either
on a set schedule or event driven basis.
 Storage: How
will the metadata be stored?
Because current digital repository systems don’t provide for the complete
range of preservation metadata, archiving institutions must create their own
mechanisms for storing the metadata. Three types of digital storage are possible:
discrete files, a database management system, and embed metadata in the objects
themselves. The discrete-file method is the simplest: metadata is stored in
text files (often using XML tagging) and associated with the digital objects
by persistent IDs in some way.[See the METS box below, for an example of an
XML framework that organizes digital objects in a tagged text file.] Database
management requires a higher level of technical commitment but has the advantage
of being capable of storing a relational model of complex objects. The third
type of digital storage, embedding metadata directly in the object, is possible
with some file formats. The TIFF format, for example, has the space and functionality
to allow metadata to be stored in the file header.
Update: When something about the
object changes, how is its metadata modified?
At a minimum, the change should be recorded as Provenance Information. If the
objects are moved to a new location, various pointers will have to be updated.
Messages to producers, users, and the repository administration might be sent
out. A desirable principle is consistency: how the changes are made and documented
should not differ within a cohesive set of objects.
METS (The Metadata Encoding
and Transmission Standard)
The METS
schema, is a flexible XML framework designed for storing
administrative, structural, and descriptive metadata about
digital objects. In addition to encapsulating the metadata
itself (or pointers to metadata stored outside the METS object),
the framework provides elements for describing the relationship
among the metadata and among the pieces of the complex objects.
What’s more, it provides tags for describing and attaching
executable behavior appropriate for content in the METS object.
In short, it is an XML-based container for all types of metadata,
for the relationships among them and the objects they are about,
and for the behaviors associated with the objects. METS’s
comprehensiveness and the flexibility designed in its structure
make it an excellent choice for a framework or container for
the objects and metadata in a preservation system.
METS is not a tool, however. An instance of METS
is an XML document. To be able to work with METS as a container
for Ingest, you need a text editor, an XML editor, or ideally,
a forms-based user interface built and customized to your collections
and to your working environment. Batch processing will require
some customized programming to integrate your metadata into the
METS structure. Using it as the container for an Archival Information
Package will also require programming work.
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Exercise |
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1. Has your organization
adopted a metadata standard that supports digital preservation?
2. Check out developing standards for metadata containers,
such as METS, MPEG-21, FOXML, XFDU.
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